DOC: Explain pypdf logging customization

[ikejourney]

Summary

• document how to customize pypdf log messages with Python's standard logging module
• show examples for handlers, propagation, and module-specific loggers
• clarify that configuring logging is preferred over patching internal helpers such as logger_warning

Closes #2020.

Checks

• ran logging snippets manually
• ran git diff --check

[stefan6419846]

Thanks for the PR.

In the issue, there were discussions about how to decrease the log level of messages using a context manager. Could we please have this in the docs as well? (Example: For my application, I want logger_warning from processing a PDF file logged at info level only.)